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Derivatives - Analysis of cash flow reserve and foreign exchange reserve (Details) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Disclosure of detailed information about hedged items [line items]    
Continuing cash flow hedge reserve £ (559) £ 292
Continuing foreign exchange hedge reserve (706) (788)
Cash flow hedge reserve, amount recognised in equity (687) 321
Foreign exchange hedge reserve, amount recognised in equity 88 (57)
Amount transferred from equity to earnings, cash flow hedge reserve (161) (50)
Amount transferred from equity to earnings, foreign exchange hedge reserve   2
Interest rate risk    
Disclosure of detailed information about hedged items [line items]    
Continuing cash flow hedge reserve (295) 695
De-designated cash flow hedge reserve (297) (424)
Cash flow hedge reserve, amount recognised in equity (700) 318
Interest rate risk | Net interest income    
Disclosure of detailed information about hedged items [line items]    
Amount transferred from equity to earnings, cash flow hedge reserve (181) (19)
Interest rate risk | Non interest income    
Disclosure of detailed information about hedged items [line items]    
Amount transferred from equity to earnings, cash flow hedge reserve 20  
Exchange rate risk    
Disclosure of detailed information about hedged items [line items]    
Continuing cash flow hedge reserve 23 22
Continuing foreign exchange hedge reserve 53 (13)
De-designated cash flow hedge reserve 10 (1)
De-designated foreign exchange hedge reserve (759) (775)
Cash flow hedge reserve, amount recognised in equity 13 3
Foreign exchange hedge reserve, amount recognised in equity 88 (57)
Exchange rate risk | Net interest income    
Disclosure of detailed information about hedged items [line items]    
Amount transferred from equity to earnings, cash flow hedge reserve (4) (35)
Amount transferred from equity to earnings, foreign exchange hedge reserve 2  
Exchange rate risk | Non interest income    
Disclosure of detailed information about hedged items [line items]    
Amount transferred from equity to earnings, cash flow hedge reserve 1  
Amount transferred from equity to earnings, foreign exchange hedge reserve (2) 2
Exchange rate risk | Operating expenses    
Disclosure of detailed information about hedged items [line items]    
Amount transferred from equity to earnings, cash flow hedge reserve £ 3 £ 4